an optional class parameter value to use when rendering the Box. The XHTML renderer for example
uses it on the surrounding SPAN (in case the box is used inline) or on the surrounding
DIV (in case the box is used standalone). This allows the user to provide CSS to style the box
according to his needs. Example: {code cssClass="mybox"}. If not specified, the
BoxMacro.getClassProperty() is used to provide a default one.